Ensure users can recover when Mermaid or LaTeX content is invalid

### User Need

To understand when diagrams or equations are incorrect and to recover without losing work or destabilizing the document.

### Goal(s)

- Users can recognize when content is invalid or incomplete
- Documents remain usable even when rendering fails
- Users can correct issues incrementally

### Non-Goals / Out of Scope

_No response_

### Examples

_No response_